Tire treads for pneumatic tires typically have running surfaces of a unitary rubber composition and therefore rubber properties attributed to the tread rubber composition across the face of the tread. The tread is usually composed of a lug and groove configuration composed of ground-contacting lugs with intervening grooves between the lugs.
Tires intended for heavy duty, in a sense of carrying large loads, such as for example truck tires, are typically intended to experience internal heat generation during the service, or operation, of the tire and to experience considerable stress at peripheral outer, or lateral, portion(s) of the tread, including tread grooves contained in the tread's ground-contacting stratified lateral zones due to, for example, vehicular cornering and tire scuffing against roadside objects including, for example, roadside curbs. When such tire stress is excessive, a surface cracking of a surface of a groove wall contained in a stratified lateral zone of the tread may occur in response to the considerable stress.
The outer, ground-contacting, tread cap rubber layer is typically comprised of a relatively low hysteretic rubber composition to promote relatively low internal heat generation as the tire is used in service as evidenced by relatively high rubber rebound and relatively low tan delta physical properties to, in turn, thereby promote a low rolling resistance of the tire tread as well as extended tread shoulder groove durability.
For this invention, it is proposed to provide the outer tread cap rubber layer in a form of circumferential zones of significantly different physical properties, particularly rubber compositions of differing physical properties such as hot rebound (100° C.) properties which are indicative of hysteresis of the rubber composition and predictive of rate of internal heat generation during use of the tire and also predictive of rolling resistance of the tire. For this invention, such tread zones are provided as an intermedial rubber zone to promote lower hysteresis with resultant lower internal heat build-up across the breadth of the tire tread positioned between and underlying lateral, stratified rubber zones.
In particular, it is proposed to provide the intermedial rubber zone, which extends across the underlying tread base rubber layer, with a higher 100° C. hot rebound property, thereby a lower hysteresis property, than the stratified overlying lateral tread zones to promote a relative maximization of reduced internal heat build-up within the tread. The stratified lateral tread zone rubber composition is therefore proposed to have a relatively lower 100° C. hot rebound property, thereby a higher hysteresis. It is further desired for the lateral tread zone rubber composition to have a greater or equal, preferably greater tear resistance property compared to the intermedial tread zone rubber, particularly to reinforce tread grooves contained in the stratified lateral tread zones.
Historically, tires have heretofore been proposed having an outer surface composed of a plurality of circumferential zones of rubber compositions to promote various properties for the tread's running surface.
For example, see U.S. Pat. Nos. 8,662,123; 7,789,117; 7,559,348; 7,131,474 and 6,959,744; Patent Publication Nos. 2007/0017617 and 2009/0107597; EP0718127, EP0798142 and DE19812934.
However, it is hereby proposed to provide a tire with tread containing a plurality of circumferential zones of rubber compositions to promote significantly different physical properties to include rebound properties to therefore promote differentiated hysteresis properties together with a peripheral sidewall extension rubber layer. The differentiated rebound properties (e.g. hysteresis properties) for the tread zones is desired to promote, or maximize, a beneficially lower internal heat buildup for the tread.
In this manner then, the central portion of the tread is a dual layered composite of an intermedial tread cap rubber zone layer and tread base rubber layer. The lateral portions of the tread are triple layered composites of the stratified rubber zones, the portion of the intermedial tread zone which extends beneath and underlies the lateral tread zones and the tread base rubber layer which underlies the intermedial tread zone.
The tire tread it thereby comprised of a cooperative layered composite of the aforesaid circumferential rubber layers.
In one embodiment, tread grooves are contained in both the intermedial tread zone and the lateral tread zones. By providing the lateral tread zone rubber layers with a tear resistance property, it is intended that tear resistance of the surface of the grooves contained in the lateral portion of the tread is promoted.
In the description of this invention, the terms “rubber” and “elastomer” may be used interchangeably, unless otherwise provided. The terms “rubber composition”, “compounded rubber” and “rubber compound” may be used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art. The terms “cure” and “vulcanize” may be used interchangeably unless otherwise provided. The term “phr” may be used to refer to parts of a respective material per 100 parts by weight of rubber, or elastomer.